Extreme Ultraviolet (EUV, 13.5 nm) imaging technology continues to be the primary option for the 22 nm microelectronics node. However, EUV resist performance remains one of the largest barriers to EUV technology implementation, because it is difficult to simultaneously meet performance targets for resolution, line width roughness (LWR) and sensitivity. For example, low concentrations of acid during imaging will yield rough lines (high LWR), but good sensitivity; high concentrations of acid will give smoother lines, but poor sensitivity. To break through to a new level of performance, new materials must be developed that will make improvements toward one performance target without compromising the performance of the other two.
Photoacid generators have been known in the polymer art for decades. Typical first-generation ionic PAGs are sulfonium and iodonium salts. In an early approach to a deep ultraviolet (DUV or 248 nm) photoresist or a 193 nm photoresist, the PAG is randomly dissolved within the polymer film. The polymer has an ester with a side-chain blocking group (e.g. t-butyl) that can be removed with catalytic acid, yielding a developer-soluble carboxylic acid. The advantage of this approach is that these resists are relatively inexpensive and simple to prepare using standard formulation methods. The resists have high sensitivity because acids are free to diffuse through the film, catalyzing acidolysis reactions (removal of ester blocking group) with large turnover numbers. The disadvantage of this approach is that the acid's rapid diffusion limits the ultimate resolution that can be achieved because the acid can diffuse into the unexposed regions of the resist—blurring the aerial image. In a second approach, the chromophore end of the PAG is bound to the polymer and the acid is free to diffuse in the film. The advantage of this approach is that the high acid diffusion rate gives these resists relatively high sensitivity, however, the disadvantage is that the high acid diffusion rate limits resolution. In a third possible approach the photogenerated acid is bound to the middle of a polymer chain and cannot diffuse very far. The advantage of this approach is that the resist's resolution will not be limited by acid diffusion. The disadvantage, however, is that the resist will have low photosensitivity, as the acid's limited movement will keep turnover numbers low.